Ongoing research and development efforts in very large scale integrated (VLSI) circuits have led to a dramatic decrease in component size and an increase in overall chip size. The increase in complexity and density of the IC's is expected to increase the speed and reliability of the systems in which they are used and, at the same time, reduce the amount of power consumed. One major limitation of packaged chips which use electrical interconnects is the relatively long distances required to interconnect devices and circuits on a common substrate or to connect chip packages on different circuit boards. Often, the interconnections use aluminum or polysilicon lines. Ohmic power losses, long delay times escessive wafer space, and complex patterning techniques, are some of the other limitations associated with electrical interconnects. Operating speed, for example, can be limited by external electromagnetic interference (EMI) from connecting lines, which can keep speeds to less than a few hundred MHz for compact systems. This greatly limits its ability for fast computing and image data processing for both military and commercial applications.
Personal computers are currently making dramatic advancements. The CPU speed has increased significantly in a short time period. Flexible USB memory key is gradually replacing floppy drives. Compact disk (CD) is advancing to CD-R, CD-RW, DVD, and DVD-RW. However, there is no significant improvement on the computer motherboard because the existing interconnect still relies on electrical means that limit its improvement potential as mentioned above.
In comparison to electrical interconnects, optical interconnects offer several advantages, including the ability to achieve high data rate signal transmission, large fanout densities, and the ability to reduce capacitive and inductive loading effects. Because of their high speed and wide bandwidth capabilities, with interconnect parameters independent of interconnection distance, optical interconnects are excellent candidates to replace electrical interconnects in a variety of applications, including those which require high data rate operation (>1 Gb/s), long distance signal propagation (intra-board chip-to-chip, board-to-board (or card-to-card), and system-to-system), low power consumption, and immunity to radiation and EMI. A comparison between optical and electrical interconnects, based on power and speed considerations, has shown that, at data speeds >3 GHz, the switching energy for electrical interconnects increases abruptly, making them unsuitable for practical systems. Crosstalk and EMI effects further degrade their performance, thereby making optical interconnects very attractive for chip-to-chip and card-to-card applications.
For card-to-card (sometimes also called board-to-board) optical interconnects, free space interconnect architecture has been extensively investigated. The use of vertical cavity surface emitting laser arrays (VCSELs) and photodetector arrays has been considered a promising approach. Although holographic based interconnection schemes and substrate backplane interconnect schemes have been examined, the preferred schemes use microlens arrays to collimate and deliver the array interconnect beams from a VCSEL array to photodetector arrays located on different circuit cards. This type of card-to-card optical interconnect has its limitations, namely it is unsuitable for interconnecting cards with optical blocking by other cards placed in between. It further suffers from alignment sensitivity since each card, after plug-in to the backplane, could be tilted in its orientation. The adjustment to achieve optical alignment is complicated by the array VCSELs and photodetector arrangement (four-axis alignment). The aligned system suffers further from vibration sensitivity. All these are the major drawbacks of current card-to-card free-space optical interconnect architectures.
The guided-wave approach for card-to-card optical interconnection offers excellent interconnect path stability and is suitable for multi-card interconnects. It is seen as the only feasible means to accommodate the large bisection bandwidth of future military and civilian processing systems. Fiber array is promising for longer distance interconnection while local channel waveguide array is suitable for shorter distance multi-card optical interconnections. The major obstacle for card-to-backplane optical interconnects using a backplane waveguide is the requirement for a 90° out of plane turn by the optical waveguide (using a 45° etched waveguide endface (mirror). This turn greatly increases the cost of manufacture and degrades backplane reliability. The out of plane waveguide coupling is not energy efficient in general and thus the systems suffer significant power loss. When some cards are not plugged in (as in the case of a computer), these 90° out of plane turns still consume optical power. This situation is different from that in an electronic backplane that does not consume electrical power when the cards are not plugged in. It is, therefore, desirable to develop an optical coupling technology with the following requirements:
Able to tap optical signal power from the backplane waveguide (or fiber) to card waveguide (or fiber) or vise versa when the card is plugged into the backplane.
The coupler is not a 90° out of plane coupler with an angle etched mirror.
When the card is not plugged in, there is no coupling power loss on the backplane waveguide. In other words, there are no terminating ends on the backplane waveguides near the card plug-in locations.
The coupler must be fabricated at low cost and the coupling performance must be reliable.
The coupler is easy to handle by any person without special training. Furthermore, it is preferred that the coupler can be operated by one hand since this would be useful for space based applications.